The present invention relates generally to filtering data in high-speed computer networks, and more specifically, to the generation and refinement of filters.
In the connected world of the Internet, destructive individuals can create major network security problems for administrators with systems exposed to public networks. The recent denial of service attacks on many of the web's most popular sites makes this clearer than ever before. A denial of service attack occurs when a malicious attacker sends continuous TCP/IP packets to a server, which quickly take up resources until there are no more resources available and a system hang or crash occurs. Commonly the targeted site may appear unavailable to the broader Internet because of the saturation of its network segment. Denial of service attacks can result in significant loss of time and money for many organizations.
Denial of service attacks are different from most other attacks because they are not targeted at gaining access to a network or information on the network. These attacks focus on making a service unavailable for normal use, which may be accomplished by exhausting some resource limitation on the network or within an operating system or application. Denial of service attacks are most frequently executed against network connectivity. The goal is to prevent hosts or networks from communicating on the network. There are several types of denial of service attacks, which can occur at various levels. When involving specific network server applications, such as a Hypertext Transfer Protocol (HTTP) server or a File Transfer Protocol (FTP) server, these attacks can focus on acquiring and keeping open all of the available connections supported by that server, effectively locking out valid users of the server or service. Denial of service attacks can also be implemented using other Internet protocols, such as UDP and Internet Control Message Protocol (ICMP).
The most common denial of service attack is the SYN attack. This is a network level attack. The attacker sends continuous TCP SYN packets to a server. Each TCP SYN packet hangs the connection until there are no more TCP resources available. The attacker begins the process of establishing a connection to the victim machine, but does it in such a way as to prevent the ultimate completion of the connection. In the meantime, the victim machine has reserved one of a limited number of data structures required to complete the impending connection. The result is that legitimate connections are denied while the victim machine is waiting to complete phony “half-open” connections. This type of attack does not depend on the attacker being able to consume network bandwidth. The intruder may be consuming kernel data structures involved in establishing a network connection. The implication is that an intruder can execute this attack from a dial-up connection against a machine on a very fast network.
High-speed networks make detecting and responding to certain types of failures and attacks difficult. The high speed makes it difficult to carefully examine every packet or even maintain state and monitor the state of every data steam without extensive hardware support. For example, a 50 MPPS switch may receive over one million flow streams per second, producing a logging data rate of 20 megabytes per second with just a 20 byte record per flow. This rate of log data is expensive to store in hardware and practically impossible to process in software. Monitoring only a subset of the traffic results in holes in detection and provides no defense against problems that exceed this subset capacity.
A more common approach is to use aggregate traffic monitoring and policing. For example, a server switch may rate limit ICMP traffic arriving on an external port to a predetermined maximum rate rather than preclude it altogether. However, an attack or a failure using ICMP may use up the entire rate, effectively blocking out other ICMP traffic, with no mechanism to determine what is causing the problem. The offending or suspicious data is therefore hidden in the aggregate. Furthermore, a high-rate attack or failure can originate upstream of a device experiencing the problem, thus compromising use of the associated link even if the device can filter out the traffic.